A sonar system is an apparatus for estimating a direction and a distance (range) of an underwater target by using sound waves.
Generally, sound waves (sound pulses or rays) are used to detect an underwater target such as a submarine. Even if the sound waves propagate with a low velocity, they are used to detect a distant target due to its less energy loss than that of an electric wave. Accordingly, a sonar system for detecting an underwater target by using sound waves for military purposes is being utilized.
The sonar system is classified into two types, a passive type and an active type.
More concretely, the passive sonar system serves to detect noise emitted from a target, whereas the active sonar system serves to detect an echo reflected from a target by emitting a sound pulse. The passive sonar system can directly detect a direction of a target in secret. However, a long time and a complicated acoustic sensor are required to detect a target range. On the other hand, the active sonar system has an advantage to simultaneously detect a direction and a range of a target within a short time even if the detection is not executed in secret.
When the sonar system is used for military purposes, detection for a target range is the most important in anti-submarine warfare (ASW). For instance, during military operations such as anti-submarine warfare, a target range has to be rapidly detected. In this aspect, the active sonar system is regarded as a system for detecting a target for a military purpose.
The active sonar system (hereinafter, will be referred to as ‘active sonar’) has a configuration shown in FIG. 1.
Referring to FIG. 1, an acoustic pulse transceiver 2 transmits a sound pulse through an acoustic sensor 1, and receives a signal reflected from a target. A signal processor 3 performs a signal process such as beam-forming with respect to the received signal. And, a detector 4 determines whether a target signal is located or not based on a signal processed by the signal processor 3. Basic information of the target detected by the active sonar indicates a direction of the target, which means an azimuth and a range. The direction of the target is detected based on a beam-forming output according to each horizontal azimuth. And, the range of the target is detected based on time taken to receive an echo after transmitting a sound pulse.
FIG. 2 shows one example of a sound velocity profile and propagation paths of a sound pulse in water. Referring to FIG. 2, it is assumed that the targets 1 and 2 are used. R(1) and R(2) indicate horizontal ranges of a sound pulse between the active sonar and the target (1) and target(2) respectively. Rpath(1) and Rpath(2) represent paths of a sound pulse between the active sonar and the target (1) and target(2) respectively.
As shown in FIG. 2, the time (‘Techo’) taken for a sound pulse is for 2-way propagation between the active sonar and targets, and consequently it is two times of the time for 1-way propagation from the active sonar to each target. Accordingly, the 1-way propagation time (‘Tsonar’) for detecting a target range is calculated by dividing the 2-way time (‘Techo’) by two.
The target range (‘Rsonar’) from the active sonar to the target is calculated by multiplying the 1-way propagation time (‘Tsonar’) with sound velocity of the pulse.
Most active sonars assume paths from the active sonar to the targets to be linear. The linear assumption between the active sonar and targets leads to easy and simple target range calculations within a short time. As a sound velocity, most active sonars employ the reference value (C0). However, if one uses just one value of sound velocity such as the reference value in water depth, one may not obtain exact target ranges at each depth.
As shown in FIG. 2, propagation paths of the sound pulse in water are not linear. Even in a case where the active sonar and the target are fixed, multi-paths are inevitably generated due to various refractions, reflections, etc. FIG. 2 just shows representative path between the active sonar and each target among many possible paths. Referring to FIG. 2, even if the targets 1 and 2 are located in different depths, the time (Tpath(1)) from the sonar to the target 1 along the path (Rpath(1)) may be equal to the time (Tpath(2)) from the sonar to the target 2 along the path (Rpath(2)). In this case, if one does not consider refraction due to the depth variation of sound velocity, one may obtain the same target range from the Tpath(1) and Tpath(2). As shown in FIG. 2, however, the target 1 range (R(1)) is substantially different from that of the target 2 range (R(2)). The conventional methods employing linear assumption have inevitable errors in detection target ranges. These errors may result in poor target range accuracy and in failure of rapid ASW operations